In 1990, a number of outbreaks of the pine sawfly, Neodiprion autumnalis, were reported in eastern Wyoming and western South Dakota. Overwintering egg populations were surveyed near Gillette, Wyoming, to assess potential for defoliation in 1991. The sawfly population appears to be collapsing at the infestation center and defoliation can be expected to be light to moderate near site 1 in 1991. Defoliation is expected to be heavy to severe up to 25 miles away from the infestation center (sites 2 and 3) with mass starvation of larvae likely. High mortality from starvation and other factors, such as parasites and predators, may contribute to collapse of the population in 1992, except perhaps for the extreme ends (north and south) of the outbreak area. Tree mortality is not likely to result from defoliation alone, but drought stress and subsequent infestation by Ips bark beetles may cause small losses.